fix: updated phpunit.xml file related to the changes in PHPUnit 9.3 (https://github.com/sebastianbergmann/phpunit/blob/a0d6b21c6c8f6564212a1a14292d230ee35eba6d/ChangeLog-9.3.md#930---2020-08-07)

Author: matthiasstern

phpunit.xml

@@ -14,15 +14,15 @@
       <directory>tests</directory>
     </testsuite>
   </testsuites>
-  <filter>
-    <whitelist>
+  <coverage>
+    <include>
       <directory suffix=".php">src/</directory>
-    </whitelist>
-  </filter>
+    </include>
+  </coverage>
   <logging>
-    <log type="junit" target="build/report.junit.xml"/>
-    <log type="coverage-html" target="build/coverage" charset="UTF-8" yui="true" highlight="true"/>
-    <log type="coverage-text" target="build/coverage.txt"/>
-    <log type="coverage-clover" target="build/logs/clover.xml"/>
+    <junit outputFile="build/report.junit.xml"/>
+    <testdoxHtml outputFile="build/coverage"/>
+    <testdoxText outputFile="build/coverage.txt"/>
+    <testdoxXml outputFile="build/logs/clover.xml"/>
   </logging>
 </phpunit>